system_region_list() could use getTheme() instead of listInfo()

  if (!$theme instanceof Extension) {
    $themes = \Drupal::service('theme_handler')->listInfo();
    if (!isset($themes[$theme])) {
      return array();
    }
    $theme = $themes[$theme];
  }

system_region_list() is retrieving info on all installed themes, when it just wants the extension object for the current theme. I'm not sure if there's a performance cost, but even if there isn't, it's clearer if the code just gets the one theme it wants.